The present invention relates generally to microelectromechanical system (MEMS) devices, and more particularly to a method of electrically isolating MEMS device structures utilizing isolation trenches filled from both sides of a silicon product wafer.
Many MEMS devices require the fabrication of electrically isolated, mechanically connected structures. One approach to realizing these structures is through the use of insulator-filled isolation trenches. Under this approach, trenches separating high-aspect ratio MEMS structures are deep-trench reactive-ion etched through the wafer being employed for fabricating the-MEMS device. After the trenches are etched, they are filled with an insulating material such as silicon nitride. This electrically isolates the MEMS structures from one another while maintaining a mechanical connection.
However, conventional methods of filling isolation trenches have significant problems with the mechanical integrity of the fill. For example, as isolation trenches are filled according to these conventional methods, insulating material accumulates at the trench openings faster than at the trench bottoms. This results in a small void near the bottom of the isolation trenches, jeopardizing the mechanical integrity of the final device. These small voids form because conventional methods of trench filling permit insulating material to be deposited from only one side of the product wafer. Accordingly, there is a need for a method to ensure uniform filling of high-aspect ratio isolation trenches.
The present invention is a method for filling a trench extending through a microelectromechanical system (MEMS) device patterned on a wafer. The method involves simultaneously depositing a trench-fill layer of insulating material over a first side of the wafer, over a second side of the wafer, and into the trench extending from the first side to the second side. Further, the width of the trench at the first side of the wafer and/or the second side of the wafer is variable to adjust the rate at which the trench fills.